Transfer of electrostatic charge pattern



May 24, 1960 L. E. WALKUP 2,937,943

TRANSFER OF ELECTROSTATIC CHARGE PATTERN Filed Jan. 9. 1957 2 Sheets-Sheet 1 21 NEGATVE POTEQTIAL L22 20 u rlllllllllllll, *1 9 I4' \\\\I//2///// g1 POSITIVE POTENTJAL POTENTIAL soufgce v HIGH 4'7 VOLTAGE SOURCE INVENTOR.

LEWIS EWALKUP y s BY May 24, 1960 l.. E. wALKuP TRANSFER oF ELECTRosTATIc CHARGE PATTERN Filed Jan. 9. 1957 2 Sheets-Sheet 2 m mzomuzz .n a omvovmmommm o m Eo. m...

nl S R3 NCUDHN 83d SJJOA dVS BIV 3H.L Nl 0131:! :$813313 mmwbm muito o wiso mmm INVENTOR. LEWIS E.WALK.UP

United States Patent O TRANSFER F ELECTROSTATIC CHARGE PATTERN Filed Jan. 9, 1951, ser. No. 633,261

s claims. (c1. tvs- 1) This invention relates in general to the formation of electrostatic charge patterns such as xerographic electrostatic latent images and to the transfer of electrostatic charge patterns between surfaces.

According to the invention of Carlson described in United States Patent `2,297,691, there is provided a process and apparatus for electrophotography or xerography wherein an electrostatic charge pattern is formed on the surface of a photoconductive insulating layer by exposure to'apattern of light and shadow to be recorded. Conventionally in the art now -known as Xerography, an electrostatic image may be formed in this manner and may be utilized as desired, for example, by development or deposition of inely divided material in conformity with .the charge pattern, optionally together with transfer of the developed image to a print receiving surface.

Recently, techniques have been developed in the art of xerography for the movement of charge patterns across air gaps and the control of charge migration in connection with the formation of or transfer of charge patterns to insulating surfaces. These newly developed techniques are described in Walkup United States Patentapplication Serial No. 368,408, filed July 16, 1953, now U.S. Patent 2,833,648. However, they are generally subject to the limitation lthat where exposure is involved thecharge pattern formed on the insulating surface is'photographically the reverse (negative to positive or positive to negative) of the original image.

Now, in accordance with the present invention, which is an improvement over the aforementioned Walltup patent application, there are disclosed methods and apparatus including an exposure and resulting in image formation on insulating surfaces in which reversal in a photographic sense does not take place4 and accordingly positives may be produced from positives and negatives from negatives without resorting to special techniques -in development or the like.

The general nature of the invention having been set forth, other objects and advantages will become apparent and obvious from the detailed discussion of the invention which follows, Vand the invention will now be described illustratively in terms of -the following specification and drawings in which:

Figure 1 is a diagrammatic view of a flow step in an embodiment of the invention;

Figure 2 is a diagrammatic view of image formation according to this invention following a step las illustrated in Figure 1;. Figure 3 is a diagrammatic illustration of a machine according to the present invention; and

Figure 4 is a set of curves defining charge transfer in accordance with the air ionization theory presently believed to underly charge migration according to this invention. .-General apparatus for implementing and carrying out the present invention is shown in Figures 1 and 2 which illustrate also the process of the present invention. According to these figures, a xerographic plate or the like, generally designated 10, comprising a photoconductive insulatinglayer or body 11 overlying a conductive backing member 12, which in this embodiment is transparent, is placed on a support 14, and the conductive backing member 12 is electrically connected to ground, either directly or through support member 14. An electrode, generally designated 16, supported on a support member 17 is positioned contiguous to, or in close proximity to, the photoconductive body 11. This electrode 16 comprises generally a `conductive electrode member 19 on which is supported a dielectric insulating member 20. Conductive electrode 19 is electrically connected through conductive lead 21 to electric potential source 22, this source being of polarity opposite to that which is ultimately desired in the electrostatic latent image which will be formed on the dielectric insulating member 20 of charging electrode 16. An illumination member 24, such as a lamp or the like, is positioned behind the transparent conductive backing member 12 of the plate 10 and is disposed to uniformly ood with illumination the photoconductive insulating body 11 through backing member 12.

In the next stage of the operation, as illustrated in Figure 2, progressing from the stage shown in Figure l to the stage shown in Figure 2, the composite body 10 remains on its support member 14 electrically connected to ground potential and the charging electrode-16cm its support 17 remains in its position in relation to the plate. However, conductive member 19 of charging electrode 16 is now connected through lead 21 to positive potential source 25. In addition, an original 35 .to` be reproduced is positioned against backing member 12 of plate 10 and is interposed between plate 10 andlamp 24. The original 35 is in this embodiment a trans-. parency and the light rays from lamp 24 pass through the transparent areas of copy 35 and then through the transparent conductive backing 12 of plate 10 to the photoconductive insulating layer or body 11.

In progressing from the step illustrated in Figure 1 to the step illustrated in Figure 2, the illumination member 24 would, in the usual case, be extinguished while the original 35 to be reproduced is positioned against the plate 10 and while the potential source 22 in Figure l is varied or changed to potential source 25.

There is also shown in Figures l and 2 the charge distribution movement now believed to accompany the operating steps of the process steps described. In the arrangement illustrated in Figure 1, illumination from lamp 24 traveling as indicated by the arrows strikes the photoconductive insulating body 11 and reduces the insulating characteristics of this body to a point whereat the photoconductive insulating member 11 may be considered conductive. Thus, the upper surface of body 11 attains the potential applied to conductive member 12 of plate 10. The negative potential applied through lead 21 from potential source 22 to conductive member 19 creates an intense electric field across the gap separating the upper surface of photoconductive body 11 and the lower surface of insulating layer 20 and when the conditions are ripe, as will be described more fully hereinafter, field discharge takes place causing a uniform positive charge to deposit on the lower surface of insulating layer, 20. The deposited charge is illustrated in this -gure by the plus signs along the lower surface of insulating layer 20. Progressing now to Figure 2, a positive potential is now supplied from source 25 through lead 21 to conductive member 19, and this applied potential tends to4 repel positive charges. Accordingly, when illumination is projected as indicated by the arrows through a' transparency to the photoconductive layer 11 of plate 10,A light passes through the transparent areas in the transparency but is stopped by opaque areas. There thus results a change in the photoconductive layer 11 makingv conductive' thoseareas' of; this layer which are in regionsV o'f illumination. There thus results a grounding of the upper surface of photoconductive layer 11 in areas of illumination and, again, when the conditions are ripe as will be described more fully hereinafter, field discharge takes place across the gap separating the lower surface of insulating layer 20 and the upper surface of photoconductive layer 11 resulting in discharge of positive charge on the surface of the insulating layer 20 in Iareas positioned adjacent to illuminated regions of the photocondu'ctive layer 11. Accordingly, following exposure as illustrated in Figure 2, there results on the surface of insulating layer 20 charge in unexposed regions following exposure as illustrated in Figure 2 of a polarity opposite to that' applied externally in Figure l. This charge pattern may be utilized through development, scanning, or the like after separating electrode 16 from plate 10 and, preferably, as will be described more fully hereinafter, separation is accomplished after exposure is completed and while a potential difference continues to be applied between conductive member 19 and backing member 12.

Reference is now had to Figure 3 wherein an embodiment of a continuous machine incorporating the principles of this invention is illustrated. In this figure, as in the previous embodiments, xerographic plate generally designated comprising a photoconductive insulating layer 11 overlying a conductive backing member 12 is supported on a support member 14 held at ground potential. Positioned above plate 10 is a conductive electrode 19, and moving between electrode 19 and plate 10 is a web of insulating material 29 supplied from supply spool 30 and fed to takeup spool 31. Web 29 is driven as through yrolls 44 which are in turn driven by driving belt 43 from motor 41. To maintain proper spacing between web 29 and plate member 10, guide and support member `48 is positioned along an edge of the plate 10 so as not to interfere with areas on which image formation takes place. Conductive electrode 19 may be supported by support member 17 or the like. Since in this embodiment the image is projected through electrode 19 and web 29, each of these must be at least substantially transparent. It is' to be realized however that the image can be projected through backing 12 if desired and in such event electrode 19 and web 29 need not be transparent. Motor 41 through belt 42 also drives rolls 38 and rolls 38 frictionally drive web 26 at a speed synchronized with the linear movement of web 29. Rolls 39 act to guide and support web 26 as it moves through the exposure area or station. Web 26 is supplied from supply spool 36 and is fed to takeup spool 37. Web 26 is illuminated by lamps 40 positioned within the shields 49 positioned and disposed to prevent light frorn lamps 40 from impinging directly on the photoconductive layer. The image on web 26 is projected through lens 27, through conductive transparent electrode 19 through transparent web 29 and to the surface of photoconductive layer 11.

The process as illustrated in Figures l and 2 has been slightly modified in the embodiment in Figure 3 by applying a charge to the lower surface of layer 29 from a corona discharge electrode comprising at least one corona discharge wire 45 positioned within shield 46. Corona discharge Wire 45 is connected through lead 34 to high voltage source 47 which supplies the corona generating potential to the corona discharge wire. High voltage source 47 may supply either positive or negative potential, the particular polarity of potential depending on such factors as the developer powders and the like which may subsequently be employed. Howenamora ever, it is to be realized that if a positive electrostatic charge is applied to web 29 a positively charged electrostatic latent image will be formed following exposure. If, on the other hand, a negative charge is applied from the corona discharge electrode, a negatively charged electrostatic latent image will be formed. Conductive member 8 held at ground potential operates to attract the generated ions from the corona discharge electrode to the surface of web 29. It is to be realized that precharging with a corona discharge electrode in this embodiment replaces precharging as carried out in Figure 1 and other similar known techniques of electrostatically charging a surface, as are known in the art, are intended to be included herein.

Conductive electrode 19 is connected through lead 21 to potential source 22 which Vmay supply positive or negative potential as is desired.

Although in Figures 1 and 2 a potential of a particular polarity is applied, it is to be realized that there is no desire to be limited thereto but, instead, particular pol'arities were used only for illustrative purposes. For example, referring again to Figure 3, if the surface of insulating web 29 is precharged with positive electrostatic charge from the corona discharge electrode supplied with positive high voltage from high voltage source 47, a positive potential may be applied to electrode 19 and the positive charge on the surface of web 29 will be discharged in illuminated areas of the photoconductive layer 11. If, on the other hand, a negative precharge is applied to web 29 from the corona discharge electrode through the supply of high negative potential from high voltage source 47, applying a proper negative potential to conductive member 19 will result in discharge of the negative charge in areas of illumination, again leaving unilluminated regions negatively charged. IIt is noted further that, although it is preferred to apply a potential to electrode 19 during exposure, the present invention will also operate with no bias on electrode 19 and through the utilization only of the field created by the precharge on web 29. The mechanism of charge migration across the gap separating web 29 and photoconductive layer 11 will be discussed more fully hereinafter in connection with Figure 4. The web 29 carrying the charge pattern is moved following exposure through a processing station as, for example, 32 wherein the charge pattern may be developed and fixed as is well-known in the art, or the image pattern may be otherwise utilized.

Reference is now had to Figure 4 wherein a chart comprising a family of curves each marked with a different voltage and also a curve designated critical stress appear. The family of curves show the electric stress for particular air gap distances applying particular applied voltages and also how electric stress varies in a gap if the gap width varies for applied voltages.

Electric stress through a material, as, for example, a uniformly thick layer of material, may be computed by dividing the voltage applied across the material by the quotient of the thickness of the material divided by thel dielectric constant of the material. When dealing with air as the layer of material separating two electrodes, the dielectric constant of air may be taken-asl l, and, accordingly, the tield applied through the air is found by dividing the voltage applied across the'layer by the thickness of the layer. Since in this invention narrow gap widths are involved which, forexample, generally are in the -order of less than microns, the stress curves are based on a micron spacing scale, and stress when read from this figure, therefore, is stated in terms of volts per micron. Further, since generally the experiments relating to this invention have been carried out in air the dielectric constant of which may be taken as 1, and since generally it is simpler to perform this invention in air as distinguished from gases and the like which might,

for example, requireclosed-off areas, electric stress in the family of curves of Figure 3 represents electricstress for the particular applied voltages across air gap dis# tances. Although the family of curves found in Figure 3 have been computed by applying a particular potential between two electrodes separated by an air gap and varying the air gap while maintaining the potential constant, it should be apparent that stress curves may be drawn taking into account any particular material, that is, in this case, any uid (gas or liquid) across which a voltage is applied. By using various potentials data to draw the family of stress curves of Figure 4 are. obtained.

The critical stress curve of this figure is drawn as a solid line along Scale 1 and also as a dotted line starting at Scale 2, which is displaced 16.9 microns along Scale 1 to account for other dielectric films in the system as will be explained'hereinafter. The critical stress curve defines electric eld strength in a gap which will sustain air breakdown. It is based on breakdown of air since the family of hyperbolas are based on field strength in an air gap. However, it is to be realized that critical stress curves for other materials may be computed if, as a matter of fact, other dielectric materials comprise the gap distance or, as will be illustrated more fully hereinafter, the curves of this ligure may be used when the other materials in the system are converted into equivalent air thickness or equivalent air gaps.

The critical stress curve of Figure 4 has been found by determining minimum applied voltages between electrodes which will support air breakdown in the gap for various gap distances, and experimentation has shown that, when the stress applied in the gap is above critical stress, transfer of charge through the gap takes place. If, on the other hand, the stress in the gap is below critical stress, dielectric breakdown will not take place. l When one is dealing with two surfaces which define the gap and which carry the applied potential, the solid line critical stress curve which starts at of Scale 1 is used. If additional material, such as the insulating material of Figures 1, 2, and 3- or the photoconductive layer of Figures 1, 2, and 3, is between the surfaces to which the voltage is applied, the voltage drops yacross these materials must be taken into account to find the eld in the gap. One technique for compensating for these materials is to position the critical stress curve at the air equivalent distance for the materials involved between the electrodes. When, for example, an insulating layer and a photoconductive layerfare in the system between the electrodes and the insulating layer is in the order of 1 mil thick having a dielectric constant in the order of 3 and the photoconductive layer is in the order of 2 mils thick witha dielectric constant of 6, since the dielectric constant of air is in the order of 1, and since the scale of electric stress curves of this chart is in terms of microns, dividing the micron thickness of the l mil insulating layer by 3, its dielectric constant, and adding to the figure found the micron thickness of the 2-rnil photoconductive layer divided by 6 provides equivalent air thickness of the vsolid dielectric material between the electrodes, which, in the example given, is 16.9 microns. Thus, and as has been done with Scale 2 in the chart of this figure, the critical stress curve is positioned to start at 16.9 microns along Scale l to take into account the effect of the solid dielectric materials (the 1-mil thick insulator having a dielectric constant of 3 and the 2mill thick photoconductive layer having a dielectric constant of 6) separating the electrodes.

It is to be realized, however, that Scale 2 in this figure is shown starting at 16.9 microns only for illustrative purposes and that it may be moved to the appropriate starting place on Scale 1 for any combination of solid dielectric materials used inthe system. When properly displaced for the solid dielectric materials in the system, and knowing the air gap between surfaces, the stress in the gapfor the particularvoltages applied can beobtained from the curves directly. ln practice, the critical stress curve is generally vplaced on a transparent member for convenience in displacing the scale to the proper starting point depending on the particular combination of solid dielectric materials being employed. This, of course, can include various thicknesses as well as various combinations of different solid dielectric materials.

Whether or not the gap will be stressed to breakdown may also be determined by finding the voltage across the gap and the gap distance and by comparing these to critical stress (the solid line curve). The voltage across the gap may be found by deducting from the applied voltage the voltage dropped across the solid dielectrics in the system, and the curves are examined in connection with the particular gap distance and gap voltage involved. For example, if an arrangement similar to Figure 1 includes an insulating layer on the electrode 16 having a thickness of 24 microns and a dielectric constant of 3, and a. photoconductive layer of the plate member of 48 microns and having a dielectric constant of 6, then the equivalent air gap spacing for the photoconductive layer and the insulating layer are each equal to 8 microns. If the air gap itself also equals 8 microns, air again being taken to have a dielectric constant of 1, and a voltage of 1200 volts is applied between the conductive member of the induction electrode and the conductive member of the plate, then the voltage would divide equally across the photoconductive layer, the air gap, and the insulating layer. This is so since these elements act as capacitors in series, and, since the capacitance of each is the same, the voltage across each would be the same.

With the backing member of the plate at ground potential, and the electrode at a positive 1200 volts, and assuming an equal division of voltage, then in a static condition we know that vthe surface of the photoconductive layer is at a positive 400 volts with respect to the plate backing member and that the surface of the insulator is at a positive 400 volts with respect to the surface of the photoconductive layer and at a positive 800 volts in respect to the backing member of the plate. To determine whether or not in the static condition described critical stress is reached, Scale 2 is positioned at zero of Scale l which provides us with the solid line critical stress curve, and the 400-volt stress curve for a gap distance of 8 microns is examined as it relates to the critical stress curve to determine whether dielectric failure results. An examination of this situation shows that sufficient voltage is applied through the gap to bring about dielectric failure within the gap and charge transfer through the gap. If, instead of going through this computation, the air equivalent thickness of the solid dielectrics had only been taken into account by dividing 48 (the micron thickness of the photoconductor) by 6 (the dielectric constant of the photoconductive material) and adding to the amount found the quotient of 24 (the mircon thickness of `the insulator) divided by 3 (the dielectric constant of the insulator), then, the air equivalent thickness of the solid dielectrics would have been found equal to 16, and the positioning of Scale 2 at 16 along Scale 1 vallows examination of the 1200volt (applied voltage) curve at a gap width of 8 microns on Scale 2 to determine whether the electric stress in the gap is above critical stress and at a point where breakdown would occur. This latter method of displacing Scale 2 along Scale 1 vhas the advantage of allowing changes in the gap distance or the applied voltage without requiring a new computation to determine the voltage across each element and, thus, simpliiies the determination of when or whether or not critical stress is attained.

Although only positive electric stress is plotted in thisl figure, there exists a similar body of negative electric stress curves and a negative critical stress curve. The

gure.

In the interest of following through the application of the information supplied by Figure 4 and for a better understanding of image. formation according to this invention, it can; be. assumed that thev photoconductive layer of the plate in Figures 1, 2, and 3 comprises a selenium layer having a dielectric. constantv of 6 and having a thickness of 50.8 microns (or 2 mils) and the insulatingv material 20 of Figures l andIZy andl 29 of Figure 3 is about 25.4 microns (or 1 mil) thick. In such a situation Scale 2. as illustrated in Figure 4 takes the solid dielectric materials into account, and, thus, it may be determined, directly from this figure (using the dotted critical stress curve) what voltages and spacings must be employed to bring about dielectric gap failure. For example, if in the arrangement of Figure l an air gap of l2 microns is. involved, the curves of this figure show that it is necessary to apply a voltage of a thousand volts or more yto bring about critical stress in the gap to cause dielectric failure of the gap. If the gap distance is increased, it is noted that less voltage need be applied between the backing of the plate and electrode 1.9 to bring about dielectric failure. For example, if the gap distance is in the order of 20 microns, then only 900 volts is necessary and at about 28 microns only about 830 volts need be applied to cause charge to transfer through the gap since critical stress through the gap will have been attained.

If the solid dielectric materials are changed, so too, changes will take place in the required applied potentials to bring about critical stress and dielectric failure in the gap. For example, if a thinner insulating layer in the order of 1/2 mil and a thinner photoconductive layer in the order of 25.4 microns (or 1 mil) having the same dielectric constants mentioned above are used, then Scale 2 would begin at 8.45 along Scale 1. In such a situation, and using the same spacingsv as previously discussed, an air gap distance of l2 microns requires only 710 volts to bring about breakdown in the gap and a gap distance of 20 microns requires only about 700 volts; whereas, 28 microns appear to require about 72() volts. It is noted that, whereas in the lirst situation of assumed values as spacing increased less applied voltage was required to bring about dielectric failure and charge transfer through the gap, yet, in the second situation of assumed values the required applied potential to break down the dielectric generally varied slightly above 700 and did not follow the same pattern of the rst assumed situation. It is noted further that, although particular techniques and dielectric constants have been assumed above, as a practical material, selenium as used in xerographic plates tends to have a dielectric constant in the order of 6 and is regularly used in xerography having a thickness varying from 20 at least through 50 microns and that insulators having dielectric constants of 3 and thicknesses of l mil are generally available. However, it is also, as should be apparent from above, rather simple to properly compensate and position the critical stress curve to take into account the particular solid dielectric material separating the electrodes to facilitate predication in a particular arrangement.

Referring now to IFigure l and assuming the solid dielectrics are of the type discussed above requiring the critical stress curve to be positioned starting at Scale 2, we find that if we apply a thousand volts or slightly less while the gap distancer between` the photoconductor and the insulating web is in the order of l microns, no charge transfer will take place since critical stress has not been attained. When the photoconductor, however, is illuminated by lamp 24 in Figure l the entire plate member may be looked upon as a conductive electrode and, accordingly, the applied potential will now appear only across insulator 20v and the air gap separating insulator 20 from plate 10. In practice it is noted that theY surface of the photoconductivey insulator when exposed to illumination would at different instances of time be at potentials tending toward the potential on the plate backing member until the surface attains the potential of the backing member at which potential it would tend to remain. 'Ihe time required for this surface to attain the potential of the backing member will depend on such factors as the light and dark decay of the photoconductive layer, the particular material the layer is composed of, the intensity and type of radiation to which it is exposed, and the like. ItA may, however, be reasonably assumed', atv least for purposes of analysis, that the potential on the backing member of the plate appears at the surface of the photoconductor, and in such a situa tion the only solid dielectric between the electrodes is the insulating layer 20. To find out the effects of the illumination of the photoconductor, the assumed thickness and dielectric constant of insulating layer 20 must be taken into account to see the effect of the thousandvolt applied potential across the assumed 10-micron air gap. In -the situation described above in which Scale 2 was employed, a l-mil thick insulating layer having a dielectric constant of 3 was employed and, accordingly, the air equivalent thickness of the solid dielectric material is equal to approximately 8.5 microns (25.4 microns, the thickness, divided by 3, the dielectric constant). The effect of the applied4 potential may now be determined either by computing the potential applied across the gap or by positioning Scale 2 at 8.5 microns along Scale l andy finding the elfect of the thousand-volt applied potential across the lO-micron gap. When this situation is examined, it is foundv that a little more than 700 volts is necessary to bring about critical stress and air breakdown in the gap and, accordingly, through illumination in. Figure 1 with, for example, a thousand volts applied charge deposition can be expected on the surface of the insulating layer with a gap of about l0 microns if the insulating layer comprises a material having a thickness in the order of about 1 mil and having a dielectric constant in theorder of about 3.

Although .the above yanalysis assumed that the surface of the photoconductivelayer attained the same potential as its backing member, it is to be realized that the same analysis may be carried out including a potential drop across the photoconductive layer during illumination. Such a situation could arise, for example, where the photoconductive layer did not act as a good conductor during exposure or the intensity or the exposure time was insuflicient for the photoconductor to attain this equilibrium condition or the like.

Turning now to Figures 2 and 3, it is noted that the insulating layer 20 or web 29 at the exposure area carries on its surface a uniform charge prior to exposure. In addition, a potential is applied externally as through electrode 19 during exposure. Thus, in a situation of this type the charge on the surface of insulator 20 or insulating web 29 must be taken into account as an additive charge to the externally applied potential or as a charge detracting from the externally applied potential, depending on the polarity of charge as it relates to the external iield applied. During exposure electric stress through the gap will vary depending on whether the gap is in an illuminated region or in an unilluminated region. Where the photoconductor is illuminated, it can again be assumed that the surface of the photoconductor in that area attains the potential applied to the backing member 12 of plate member 10. However, in. unilluminated regions the photoconductive insulating material remains insulating and, accordingly, a potential drop will appear across the photoconductive insulator 11. Thus, if in the. same arrangement discussed in connection with Figure l a thousand volts is applied using a lO-micron gap distance and the thousand volts applied takes into account takes into account the charge on the surface of insulator 20 or insulating web 29 it can be expected that without exposure no charge transfer or substantially no charge transfer will take place. However,v when as illustrated in Figures 2 and 3, the assembly is exposed to alight pattern or other activating radiation pattern, for example, X-rays, ultraviolet radiations, or the like as is well-known in the art, the photoconductor in the illuminated sections vbecomes conductive thereby placing a greater potential drop across the gap between insulator A20 or lweb 29 and the photoconductive layer and a sufficient field to cause transfer or charge migration through the gap in exposed regions. Accordingly, following exposure when the conditions are right for charge transfer as defined in Figure 4 there will result on the surface of insulating layer 20 or insulating web 29 an electrostatic latent image conforming to the pattern of light and shadow to which the photoconductor has been exposed and conforming further by having charges deposited in regions which have not been exposed and no charge deposited in regions which have been exposed or, stated more accurately, having a greater charge density in areas of no exposure and lesser charge densi-ty in areas of exposure.

Itis also noted, as may be seen in connection with Figure 4, that a spacing or gap between the insulator on which the image is to be formed and the photoconductive layer can be too small to allow charge migra- Vtion between the surfaces. ln such event, if a precharged insulator is placed against the' photoconductor at, assume', 1 to 2 microns of spacing, and exposure takes place, a charge pattern will appear at the upper surface of the photoconductive insulator. This pattern will be an induced pattern drawn to the surface by the electrode. Following exposure the photoconductor will again become insulating in all areas that were exposed and, ac-y cordingly, the pattern will remain in place on the surface ofthe photoconductive insulating layer. By maintaining the potential on electrode 19 during separation a gap 'distance can be attained for the applied voltage to cause charge transfer in image areas and, thus, discharge of the precharged surface of insulator 20 or web 29 in accordance with the image pattern. It is quite apparent that if, for example, a thousand-volt potential difference appears across the gap in areas of image at l or 2 microns this potential difference will not be sufficient to bring on critical stress and charge transfer. However, as this gap distance increases and the potential is maintained through the continuing application of lthe external source 25 or 33 in Figures 2 and 3 a gap distance willl bereached so that charge will transfer resulting in discharge of areas on the insulating surface. Charge transfer to form an image during separation is intended to be included in this invention as well as other modifications of this nature 'which will readily occur to those skilled in the art.`

' When critical stress is attained, ions which normally are present in the gap are accelerated into collisions with nearby air moleculesv thereby creating additional ions which similarly collide with molecules to create more ions, etc. Also, charges are released from the surfaces defining the gap by collisions with the surfaces by the moving ions in the gap creating additional ions in the gap, and the created ions traveling in the space between the surfaces deposit on thesurfaces controlled by the electric field, thereby producing the transfer of charge or the deposition of charge. The ions created, it is to be realized, are both positive and negative, and the positively Charged ions move to the negative surface, whereas the negatively charged ions move to the positive electrode resulting in neutralization vof charges which exist on the respective electrode and also resulting in the deposition of new charges raising the amount of charge deposited on the'velectrode surface. For example, where there exist negative charges on a surfacer andpositive charges are moved ,tothe surface,pneutralization takes place and thed charge density of the negatively charged surface is. ref; duced. I fon theother hand, a surface isv substantially t' `a neutral potential and charges are moved tothe surface by the electric field in the gap,, then deposition of additional or new charges takes place onl the surface without neutralization, thereby raising the charge density on the previously neutral surface as controlled by the field. Thus, air ionization which ltakes placein the gap in a real sense creates a conductive gap and allows charge flow between the surfaces defining the gap, thereby result-v ing in charge migration between the gap surfaces. This type of air ionization and air travel continues while the electric stress in the gap is above critical stress and for a slight stress below critical stress once current flow through ion movement in the gap has started, and as depositiontakes place the electric stress in the gap is reduced by the deposited charges which change the electric field strength across the gap until charge migration stops. Since charge is neither destroyed nor created within the gap area, and since charge does not leak out of the gap, an equal amount of charge deposits on each surface defining the gap as controlled by the electric field in the gap.

Thus, if the two surfaces defining the gap are insulating, charges which transfer to either surface remain in position thereon. Accordingly, if one is dealing, for example, with 300 volts above critical stress, charge will deposit on each surface to vary each surface by volts (one surface being varied upward and the other downward) thereby dropping the stress in the gap beneath critical stress. If one of the surfaces defining the gap, on .the other hand, is conductive and is maintained at a particular potential, then rather than transferring half the potential difference above critical stress to the adjacent insulating surface, the entire potential difference above critical stress will transfer in' that the charges drawn to the conductive electrode do not lower the field in vthe gap, and it is only charges which deposit on the insulating surface which reduce the field applied across the gap or the electric stress in the gap. Accordingly, sufiicient charge must depositen the insulating surface to bring the gap stress below critical stress, and, since the potential on the conductive surface of the gap remains the same, sufficient charge must deposit on the insulating surface defining the gap to reduce the potential difference between the insulating surface and the conductive surface to below that of critical stress. It is noted that in the embodiment, for example, illustrated in Figure l where charge uniformly transfers across thev gap during exposure of the photoconductor, if the photoconductor is a material which during exposure becomes a good conductor, it can be expected that the entire amount of potential applied above critical stress will transfer across the gap to the insulating surface. Similarly, during exposure in Figures 2 and 3 in areas exposed if the photoconductor becomes a good conductor it can be expected a charge sufficient to reduce the potential difference across the gap to below that of critical stress will transfer from the insulating surface.

Although in some situations following charge transfer and image formation the externally applied potential can be removed and the surfaces can be separated, it is gen` erally desirable to maintain the externally applied potential until the surfaces are apart from one another. If charges transfer during separation the externally applied field is valuable as an aid to uniformity of charge transfer during separation. In addition, after charges have transferred whether during image formation as, for ex ample, illustra-ted in Figure 2 or Figure 3 or during separation, if the externally applied field is removed, then at least in image areas there will exist a field across the gap due to a difference in charges on the surfaces deffining the gap. If the surfaces are separated from one another withou-t an externally applied field the capacitanceV between the surfaces decreases, and, since potential,

between surfaces of a capacitator increases as capaci- ',k

tance decreases, the potential difference can becomejso intensified as to cause air breakdown in an uncontrolledmanner. Accordingly, to avoid air breakdown between the-surfacesas well as laterally along the 'surfaces it is generally preferred to maintain the externally applied potential until Icomplete separation of the surfaces has taken place.

In carrying out this invention while the surfaces are separated, spacings in the order of to 30 microns are presently preferred and optimum spacing appears to be at about 2O microns. Although charges will transfer through both narrower and wider gaps, it is to be realized that as the gap becomes narrower it is more difficult to maintain uniformity of gap distance between the surfaces and as the gap becomes wider the image transferred tends to spread thereby irnpairing image quality. Such narrow gap distances may be maintained as through spacers, particles deposited on the surface, and the like. In addition, it is submitted that as a practical matter no real concernneed be had with maintaining the gap distance if the surface of the insulating layer to be placed adjacent to the surface of the photoconductive layer is precharged as through the use of corona or the like and then placed in intimate contact with the photoconductive layer during exposure to an image pattern. Following exposure and with the externally applied potential still applied the adjacent surfaces can be separated from one another and charge transfer will take place during separation, if not previously, thereby resulting in the desired image -on the insulating layer.

,The term insulating as used herein, whether in connection with the photoconductor or in connection with the image transfer sheet, is intended to refer to materials which have sufficiently high resistance under conditions of use to holdan electrostatic image for a period which permits utilization of the image by transfer to another surface or by development. It is obvious that a lower resistance can be tolerated with a more rapid processing cycle. Image transfer materials which have been used in carrying out this invention have included plastic layers such as polyethylene, terephthalate, dried or coated papers or the like generally having a resistivity in the order of above 1012 ohm centimeters.

To date, although many materials which can be used as the photoconductive layer or as the insulating transfer layer have been investigated, not all possible materials have been examined. Thus, although experimental work tends to prove that the theory of gas ionization accounts primarily for the transfer of charges (and, for this reason, the gas ionization process has been discussed in great detail) eld emission, secondary emission, and the like may also enter into operation in connection with this invention, depending on various factors. For this reason, the term field discharge is used in the specification and claims and is intended to mean that form of limited discharge within the scope of this invention which results in charge transfer to an adjacent surface through a gap and in true conformity to an original charge pattern being formed on an image forming surface such as a photoconductive layer or in true conformity to an original charge pattern existing on a surface in which transfer is brought about through the application of intense electric fields through short fiuid gaps, which fields, however, are not intenseenough to creat spark discharge or because. of the close proximity of the adjacent surfaces defining the gap, spark discharge in the usual sense is prevented due to the limitations placed on the paths of travel of the ions in this space.

When such other phenomena as field emission or the like come into operation, slight variations from the predictions which are possible under the air breakdown theory described in detail in this specification can be expected. However, it is noted that for many materials sufficient information is available to determine when field emission, for example,jwill take place, and if the field emission curve for the particular material is positioned on the curves illustrated 'in Figure 4, knowing the potential applied allows one to determine whether field emission will come into operation or whether, as a matter of fact, air ion'- ization only accounts for charge transfer. For example, where transfer of charge results with a gap width of less than 2 microns, it can be expected that a mechanism other than air ionization accounts yfor transfer. lif field emission alone or in combination with other known mechanisms of operation account for transfer, new curves may be drawn for the particular system, thereby allowing complete predictability for the system. It is noted further that other phenomena, not all presently known, also seem to cause slight variances from the air ionization theory. For example, it has been found that when amorphous selenium is used as the photoconductor, variations from theory result which are presently believed to be a form of clean-up process within the selenium layer in which carriers are first swept from the selenium, thereby establishing conditions allowing charge to transfer in accordance with theory. However, even though results in connection with this invention may vary slightly from the theoretical predictions which can readily be made following air breakdown theory, the air breakdown theory is extremely helpful in setting up predictable results which, as a practical matter, will be different from experimental results only by an amount which might otherwise be considered experimental error. Yet, this variation in results, it is to be realized, is due to the other phenomena which are included with air breakdown in the term field discharge, which also come into play during charge transfer across a gap while intense electric fields are applied across a narrow gap.

The photoconductive insulating layer is, in the preferred embodiment of the inventon, a vitreous or amorphous selenium layer, preferably deposited on the surface of a conductive backing member. Members of this type comprising a vitreous or amorphous selenium layer are commercially available in the art of xerography. Inad dition to this specifically preferred embodiment, it is to be understood that there may be employed other photoconductive members including anthracene, sulphur, and the like, coated on suitable backing members, as well as photoconductive binder layers including photoconductive materials dispersed in a suitable binder and coated on a conductive surface. These materials include, for example, crystalline materials generally available as phosphors or luminous compounds which frequently exhibit photoconductivity and which can be employed in suitable organic binders land film forming materials on the surface of photoconductive members. It is noted, in addition, that, although at times some of these materials act as photoemissors, photoemissive properties are not being utilized in this invention due to the quick disappearance of such properties when the materials are used in air, other gases, or the like.

In the case of photoconductive layers, according to the present invention,` as distinguished from the photoconductive insulating layers preferred in the art of xerography in general, it is to be observed that the layers according to the present invention are not required to hold an electrostatic charge on their surface for similarly long periods of time but are required, on the contrary, to permit migration of electric charge through the thickness of the layer at a substantially differential rate depending on activation or nonactivation by suitable radiation. Thus, while photoconductive layers for xerography in general must be capable of supporting an electric charge on the surface yfor appreciable periods of time to permit formation of an image and development of the image, this is not true in the case of the present invention where the electrostatic image is retained for development on the insulating layer. Thus, xerographic members characterized by higher dark decay or current leakage in the absence of activating radiation may be employed in the pres-` ent invention.

This invention has been described as carried out in the gamas t t 13 t specificembodiments thereof and it is not desired to be limited thereby, but is intended to cover the invention broadly within the spirit and scope of the appended claims. p v

` This is a continuation-in-part of patent application Serial No. 368,468, filed July 16, 1953, entitled Transfer of Electrostatic Charge lattern, now U.S. Patent 2,833,- 648.

What is claimed is:

l. The method during which a xerographic plate comprising a photoconductive insulating layer overlying a conductive backing member is exposed to an image pattern while the plate backing member is maintained at ground potential of forming an electrostatic charge pattern on the surface of an insulating layer comprising positioninga uniformly electrostatically charged surface of the insulating layer, charged to a raised potential in respect to ground potential and charged to a first polarity in respect to ground potential, adjacent to and facing said photoconductive insulating layer of said plate, applying an electric field through the photoconductive insulating layer and to the adjacent and facing surface of the insulating layer by applying a potential in addition to the potential difference applied between the electrostatically charged facing surface of said insulating layer as it relates to said grounded backing member through the application of a potential of said first polarity relative to'ground across the surface of said insulating layer opposite to said facing surface while simultaneously exposing the photoconductive insulating layer to an image pattern, and separating the adjacent and facing surfaces of said insulating layer and said photoconductive insulating layer from one another, said applied electric field created by said applied potentials being of a suiiicient intensity to bring about field discharge by applying electric potentials above critical stress between the facing surfaces at least prior to complete separation of the facing surfaces from one another to thereby dissipate charge from the insulating surface in areas contiguous with areas of the photoconductive layer activated by the image pattern.

2. The method during which a xerographic plate cornprising a photoconductive insulating layer overlying a conductive backing memberis exposed to an image pattern while the plate backing member is maintained at ground potential of forming an electrostatic charge pattern on the surface of an insulating layer comprising positioning a uniformly electrostatically charged surface of the insulating layer, charged to a raised potential in respect to ground potential and charged to a first polarity in respect to ground potential, adjacent to and facing said photoconductive insulating layer of said plate, applying an external electric potential difference through the photoconductive insulating layer and at least to the adjacent and facing surface of the insulating layer by applying a potential in addition to the potential difference applied between the electrostatically charged facing surface of said insulating layer as it relates to said grounded backing member through the application of a potential of said first polarity relative to ground across the surface of said insulating layer opposite to said facing surface while simultaneously exposing the photoconductive insulating layer to an image pattern, separating the facing surfaces of said insulating layer and said photoconductive insulating layer from one another while vthe external potential difference remains applied, said applied electric potential differences being of a sufficient intensity to bring about field discharge by applying electric potentials above critical stress between the facing surfaces at least prior to cornplete separation of the facing surfaces from one another to thereby dissipate charge from the insulating facing surface in areas contiguous with areas of the photoconductive insulating layer activated by the image pattern, and then removing the externally applied electric potential difference.

3. The method during which a xerographic plat@ comprisinga photoconductive insulating layer overlying a conductive backing member is exposed to an image pattern while the plate backing member is maintained at ground potential of forming an electrostatic charge pattern on the'surface of an insulating layer comprising placing a uniform electrostatic charge across the surface of an in.- sulatingV layer to raise the surface to an elevated potential in respect to ground and to a potential of a first polarity in respect to groundpositionng said uniformly electrostatically charged surface of the insulating layer adjacent to and facing said photoconductive insulating layer of said plate, applying an electric field through the photoconductive insulating layer and to the adjacent and facing surface of the insulating layer by applying a potential in addition to the potential difference applied between the electrostatically charged facing surface of said insulating layer as it relates to said grounded backing member through the application of a potential of said first polarity relative to ground to an electrode adjacent to the insulating layer and across the surface of said insulating layer opposite to said facing surface while simultaneously exposing the photoconductive insulating layer to an image pattern, and separating the facing surfaces of said insulating layer and said photoconductive insulating layer from one another, said electric fields created by said applied potentials being of a sufficient intensity to bring about field discharge by applying electric potentials above critical stress between the adjacent surfaces at least prior to complete separation of the surfaces from one another to thereby dissipate charge from the insulating surface in areas contiguous with areas of the photoconductive layer activated by the image pattern.

4. The method during which a xerographic plate comprising a photoconductive insulating layer overlying a conductive backing member is exposed to an image pattern while the plate backing member is maintained at ground potential of forming an electrostatic latent image on the surface of an insulating layer comprising positioning the insulating layer adjacent to a photoconductive insulating layer but spaced apart therefrom by a small fluid gap with a facing surface of said insulating layer defining one side of said gap, applying an electric field through the photoconductive insulating layer and to the adjacent surface of the insulating layer by applying a potential of a first polarity in respect to the grounded backing of said plate across the surface of the insulating layer opposite to the surface of the insulating layer facing said photoconductive insulating layer, flooding the photoconductive insulating layer with uniform illumination, said applied potential being suiciently intense to stress said gap above critical stress at least during illumination to result in uniform charging of said facing surface of said insulating layer, extinguishing the illumination, reversing the polarity of the field applied through the photoconductive insulating layer and to the adjacent surface of the insulating layer by applying an electric potential of a second polarity in respect to ground across the surface of said insulating layer opposite to said facing surface, and exposing the photoconductive insulating layer to an image pattern, said applied second polarity electric potential being of an intensity to raise the stress above critical stress between said surfaces for the particular gap between them to at least cause field discharge in the gap in areas corresponding to said photoconductor activated by said image pattern to thereby form an image pattern to discharge the charge on the surface of the adjacent insulating layer conforming to areas of the photoconductive insulating layer activated by the exposure image pattern.

5. The method during which a xerographic plate comprising a photoconductive insulating layer overlying a conductive backing member is exposed to an image pattern while the plate backing member is maintained at ground potential of forming an electrostatic latent image on the surface of an insulating layer comprising positioning the insulating layer adjacent to a photoconductive insulating 15 layer but spaced apart therefrom by a smallair gap with a facing surface of said insulating layer defining one side of said gap, applying an electric eld through the photoconductive insulating layer and to the adjacent surface of the insulating layer by applying a potential of a rst polarity in respect to the grounded backing of said plate across the surface of the insulating layer opposite to the surface of the insulating layer facing said photoconductive insulating layer, uniformly illuminating the photoconductive insulating layer, said applied potential being suiciently intense to stress said gap above critical stress at least during illumination to result in uniform charging of said facingsurface of said insulating layer, extinguishing the illumination, reversing the polarity of electric field applied through the photoconductive insulating layer and to the adjacent surface of the insulating layer by applying an electric potential of a second polarity in respect to ground across the surface of said insulating layer opposite to said facing surface, exposing the photoconductive insulating layer to an image pattern while the reverse field remains applied, said applied second polarity electric potential being above critical stress between said adjacent surfaces in view of the gap present therebetween to at least cause field discharge in the gap in areas corre- 16 sponding to said photoconductor activated by said image pattern to result in discharge of charge on the facing surface of the adjacent insulating layer conforming to areas of the photoconductor exposed to said exposure image pattern, and separating the adjacent surfaces while the second polarity applied potential remains applied.

References Cited in the le of this patent UNITED STATES PATENTS 2,221,776 Carlson Nov. 19, 1940 2,297,691 Carlson Oct. 6, 1942 2,693,416 Butterfield Nov. 2, 1954 2,758,525 Moncrieif-Yeates Aug. 14, 1956 2,808,328 Jacob Oct. 1, 1957 2,825,814 Walkup Mar. 4, 1958 2,833,648 Walkup May 6, 1958 2,833,930 Walkup May 6, 1958 FOREIGN PATENTS 1,105,940 France Dec. 9, 1955 OTHER REFERENCES Non Destructive Testing, vol. 10, No. 1, 1951, page 14. 

1. THE METHOD DURING WHICH A XEROGRAPHIC PLATE COMPRISING A PHOTOCONDUCTIVE INSULATING LAYER OVERLYING A CONDUCTIVE BACKING MEMBER IS EXPOSED TO AN IMAGE PATTERN WHILE THE PLATE BACKING MEMBER IS MAINTAINED AT GROUND POTENTIAL OF FORMING AN ELECTROSTATIC CHARGE PATTERN ON THE SURFACE OF AN INSULATING LAYER COMPRISING POSITIONING A UNIFORMLY ELECTROSTATICALLY CHARGED SURFACE OF THE INSULATING LAYER, CHARGED TO A RAISED POTENTIAL IN RESPECT TO GROUND POTENTIAL AND CHARGED TO A FIRST POLARITY IN RESPECT TO GROUND POTENTIAL, ADJACENT TO AND FACING SAID PHOTOCONDUCTIVE INSULATING LAYER OF SAID PLATE, APPLYING AN ELECTRIC FIELD THROUGH THE PHOTOCONDUCTIVE INSULATING LAYER AND TO THE ADJACENT AND FACING SURFACE OF THE INSULATING LAYER BY APPLYING A POTENTIAL IN ADDITION TO THE POTENTIAL DIFFERENCE APPLIED BETWEEN THE ELECTROSTATICALLY CHARGED FACING SURFACE OF SAID INSULATING LAYER AS IT RELATES 